I've just purchased a couple of second hand Athearn RTR GP40s (the old style with no DCC board) and I'd like to fit them with sound decoders. These will obviously need to be hard wired types.

I've never fitted sound decoders before and only have one factory fitted sound decoder in an Athearn Genesis GP15-1. I'd be interested in recommendations and experiences for the most effective sound decoders for both sound qualityy and motor control.

I would always recommend the Loksound Select or V4. The motor control is wonderful, straight out of the box, and rarely needs any tweaking. The sounds can also be upgraded when new or better are released, and are in some cases better than Tsunami. The horn sounds are almost all much better.

Tsunamis can be made to run smoothly, but there can be a huge amount of faffing about to get you there.

Also a Loksound fan.
Go with a Select to have something similar to a Tsunami, but cheaper at £75, better motor control, 6 function outputs, fully re-mappable functions and downloadable sound projects and firmware updates. The programmer is required to change projects and firmware, but the functions, sound selections etc. are done with CVs.

A V4 also gives the ability to change the individual sounds, completely alter the way the sounds work and make you own sound projects.
Functions 5 & 6 need extra components.
The programmer is required for all the sound work.